This invention relates to explosion suppression and isolation apparatus for use with structure which confines highly combustible, flowable material and that is normally conveyed to or from a collection or processing area remote from the structure through an interconnecting conduit. The combustible material presents a hazard in that flame and combustion generated pressures resulting from an unforseen ignition and explosion of the material will rapidly and often destructively be directed into the processing or collection area.
The apparatus hereof is operable to prevent the propagating flame front from an explosion transitioning from a deflagration state to a detonation state, and to then isolate and prevent the suppressed flame and deflagration pressures from entering the collection or processing area through the conduit.
Many industrial processes involve handling of highly combustible and therefore very hazardous materials, which are normally confined within containment structure, but are then directed through an interconnecting conduit to another processing or collection area. Exemplary in this respect are machining operations on aluminum and magnesium products which produce very small metal fines. The machining operation often is carried out within structure which confines the metal particles, or the resulting fines can be directed into a vessel for storage until the material is delivered through a conduit or the like to a desired collection point or processing area. Similarly, extremely hazardous, flammable fluids or gases are received or stored in a confinement vessel which is also connected to a processing or collection area by a conduit.
The collection or processing area which receives the hazardous metal fines, other types of solid, very small combustible particles, or combustible gaseous or fluid materials is usually spaced some distance from the initial storage or confinement vessel. A conduit is most often used to convey the hazardous flowable material from the containment or storage structure or vessel to the point where it is either collected or further processed.
If the highly combustible material in the containment structure or storage vessel ignites as a result of an unforseen event, the propagating flame front resulting from the ignition rapidly transitions from an initial deflagration state to a detonation state within the conduit. Undesirable flame and often destructive pressures may therefore be delivered directly into the collection or processing area through the conduit which connects the containment structure or storage vessel with the collection or processing area.
Typically, in view of the volume of highly flammable, flowable materials that must be appropriately contained and then directed via a conduit to a collection or processing area remote from the point of collection or containment, the delivery conduits are of relatively large diameter, e.g., 12 to 24 in. Furthermore, the conveyance structure which for example may comprise of a delivery conduit often includes bends or other obstacles which induce turbulence that substantially contribute to the acceleration of flame propagation. Ignition of combustible material may occur in the confinement structure which also substantially contributes to acceleration of flame propagation by a rapid injection of flame into the interconnecting conduit or pipe. In view of the violent nature of explosions that may occur from containment, storage and conveyance of highly flammable materials as described, as well as others having similar hazardous characteristics, there has been no reliable way to prevent flame transition to detonation and isolation of the combustion flame and pressure from the explosion so that the flame and pressure wave do not enter the defined collection or processing area.
It has been proposed to protect a processing or collection area which normally receives the highly flammable material from the containment structure or storage vessel, by providing equipment for directing a suppressant agent into the material-conveying conduit downstream of the containment structure or vessel. A detector in that proposal is located to sense ignition of the combustible material ahead of the location where a suppressant agent is delivered into the conduit. In the case of deflagrations of highly flammable materials originating in the containment area adequate suppressant agent cannot be effectively delivered to a large diameter delivery conduit at the necessary rate and for a duration to prevent transition of the deflagration to a detonation state. Likewise, it has not heretofore been feasible to mechanically block entry of flame and combustion generated pressure produced by an explosion of highly combustible material from entering the collection or processing area to be protected when the deflagration has transitioned to detonation velocities. Conversely, it is not possible to place a mechanical isolation device at a location ahead of the distance where a deflagration can transition to a detonation and still provide sufficient time to effect closing of the valve.
In order for an explosion to occur, a fuel and oxidizer mixture within the flammable limits of the fuel must be exposed to an ignition source of adequate strength to initiate combustion. If the flammable material is contained in a structure or is in an elongated pipe or conduit, immediately upon ignition, an explosion will propagate from the ignition point into the unburned fuel and oxidizer mixture. A spherical flame front is first formed which continues to grow until the confining walls are reached. A pressure wave is also generated, which travels at the speed of sound of the mixture it is propagating into. At this point in time, the flame front and the pressure wave are traveling at different speeds, with the pressure wave traveling much faster than the flame front.
Once the flame front has reached the wall of the pipe or conduit, it changes from spherical form to an essentially planar front. As the planar flame front continues to propagate down the length of the pipe, it begins to elongate and the surface of the flame increases. As the surface area increases, the burning rate increases and as a result, the flame propagation velocity increases. This stage of an incipient explosion initially involves a phenomena known as xe2x80x9cdeflagration,xe2x80x9d which may be defined as conditions where the pressure wave and flame front are traveling separately, the pressure wave is traveling at the speed of sound, and flame front propagation involves heat transfer.
The pressure wave and the flame front eventually coalesce into a shock wave. If propagation continues, the energy of the pressure wave is sufficient to cause localized explosions. At the point where the pressure wave is strong enough to initiate the combustion reaction, the explosion phenomena becomes known as xe2x80x9cdetonation.xe2x80x9d In the initial stages, the detonation wave will propagate into a precompressed mixture of fuel and oxidizer, known as xe2x80x9cover-driven detonations.xe2x80x9d The over-driven detonation will catch up to the foremost pressure wave and become a stable detonation with a constant velocity. A stable detonation wave consists of a pressure wave closely coupled with a flame front such that the energy released by the flame front supports the pressure wave.
Therefore, in a typical explosion in a conduit, the deflagration stage may be followed immediately by detonation. At each stage of an explosion, magnitude of pressure, rate of pressure rise, flame velocity and relative location of flame front to the pressure front, are different, depending upon the material that is susceptible to exploding, the point of ignition, and the nature of the conduit along which the flame is propagating.
In the deflagration region of an incipient explosion, pressures experienced increase from 0 bar g up to no more than about 10 to 12 bar g. In the detonation region, pressures can varying from about 20 up to as much as 80 bar g. Flame velocity in the deflagration region is usually of the order of 100 to 300 m/s, while flame velocity in the detonation region typically will rise to a level of about 1500 to 2500 m/s.
The size of the particles of the combustible flowable material has an effect on the overall explosion phenomena, as does the diameter of the conduit through which the products of combustion are flowing. Pipes of larger diameter provide smaller heat sinks than smaller diameter pipes or conduits. The longitudinal configuration of the conduit also affects the propagation phenomena. Obstacles and bends in the pipe or conduit can exert turbulence which in turn will effect flame surface area and cause faster transition to detonation. Where ignition occurs in a closed vessel or containment structure, it is known as xe2x80x9cprevolumexe2x80x9d ignition. This leads to initially higher flame propagation speed, faster transition to detonation, and higher pressure generation.
The goal of explosion protection is to suppress the deflagration stage of the explosion, preventing the deflagration phenomena from transitioning into detonation phenomena, and block the flame and combustion generated pressures from entering a protected area at the end of the conduit or pipe opposite the containment structure or storage vessel that normally receives the hazardous material. In the case of highly flammable and hazardous flowable materials such as aluminum and magnesium particles, other similar metal fines, or gases such as hydrogen, this goal has not heretofore been realized.
The present invention provides deflagration suppression and explosion isolation apparatus for preventing flame and combustion generated pressures resulting from explosion of a highly flammable, flowable material in a containment structure or a storage vessel from entering a collection or processing area that normally receives the material or is the sources of the material via a conduit interconnecting the structure or vessel and the collection or processing area.
The overall deflagration suppression and explosion isolation system includes containment structure, which may for example comprise a storage vessel or compartment, for confining a flowable, highly combustible material which presents a fire and explosion hazard, such as aluminum or magnesium dust, certain highly flammable organic materials, and gases such as hydrogen. An elongated conduit connected to the structure normally conveys flowable material to or from the structure to a collection or processing area remote from the containment structure. The conduit is typically of a length and configuration longitudinally thereof that upon unforseen ignition of the material in the structure, flame can course along the conduit in the form of a deflagration front that transitions into a detonation state before reaching the material collection or processing area.
A suppressant device communicating with the conduit is in disposition to direct a fire suppressant agent into the conduit. A detector associated with the structure and conduit is operable to sense ignition of the material in the structure and to activate the suppressant device to deliver suppressant agent into the conduit. The suppressant unit is located on the conduit along the length thereof in disposition to begin introducing suppressant agent into the conduit before the flame has reached its location.
An isolation assembly connected to the conduit ahead of the collection and processing area and after the suppressant unit is operable in association with the suppressant device to ;prevent flame and combustion generated pressure from entering the collection or processing area via the conduit. Isolation of the collection or processing area is preferably accomplished through provision of a gate valve connected to the conduit downstream of the suppression agent delivery device which has a valve plate normally in unblocking relationship to the conduit, but that can rapidly move into a position fully blocking the conduit upon detection of an incipient explosion by the ignition detector. In a preferred form of the invention, the suppressant device includes a vessel for storing a quantity of a powder suppressant under gas pressure, a rupture disc normally preventing release of suppressant from the suppressant vessel, and a gas cartridge unit operable to produce a gaseous discharge sufficient to rapidly rupture the disc upon receipt of an activation signal from the incipient explosion detector. In that same preferred form of the invention, the gate valve also is provided with a gas cartridge unit which is operable to produce a gaseous discharge which effects rapid closing of the gate of the gate valve when the incipient explosion detector detects ignition of the highly flammable material in the containment structure.